And remaining residual allows the conclusion to be made that the residual must

be higher than about 10–15% of the peroxide input in order to achieve the best

Result. As a lower residual results in a poorer gain in brightness, it is therefore

Bleaching of Mechanical Pulp

1 1.4 1.8 2.2 2.6 3

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Peroxide residual [%]

NaOH (%)

1,0 1,5 2,0 2,5 3,0

Brightness [% ISO]

NaOH-charge [%]

Fig. 6.3 Impact of the variation of the caustic soda charge on

brightness. Bleaching of softwood TMP with 4% H2O2 and

2% sodium silicate, constant: 25% consistency, 3 h, 70 °C.

Important to control the peroxide residual and to evaluate the ratio between residual

And input.

Bleaching response is improved with consistency. The reason for this is the

Higher relative concentration of the chemicals and the lower level of dissolved

Compounds. The steep increase in bleaching efficiency with the consistency is

Visualized in Fig. 6.4. Because of the importance of high-consistency, modern

bleach plants operate well above a level of 25%.

The addition of sodium silicate in bleaching has several effects. First, it acts as a

Buffer, and therefore reduces the peak value of the pH. The instability of diluted

0 5 10 15 20 25 30 35

Brightness [% ISO]

Pulp consistency [%]

Fig. 6.4 Impact of consistency on mechanical pulp bleaching.

Constant application of 1.5% H2O2, 3% sodium silicate at

C, 2 h, amount of NaOH adjusted.

Bleaching with Hydrogen Peroxide

Silicate solutions favors the precipitation of silicates (silicic acid sols), and this can

Lead to the removal of potential decomposing compounds. Because such precipitation

Can cause problems with scaling, the amounts of silicate added are typically

Adjusted to lower levels. Despite these difficulties, the advantage of silicate addition

normally keeps this product in the mill’s receipt. A clear example of the benefit

Is shown in Fig. 6.5, where silicate addition leads to a higher brightness and a

Wider maximum. In the presence of silicate, the receipt for composition of the

Chemicals tolerates deviations more easily. The standard addition of silicate is

around 2%, calculated as commercial solution with 38–40 Be density. Silicate solutions

With a higher alkalinity have a lower average molecular weight for the silicic

Acid polymer, and this permits a lower level of addition, with identical effects.

1,0 1,5 2,0 2,5

Na silicate 0 % 1.5% 3%

Brightness [% ISO]

NaOH-charge [%]

Fig. 6.5 Impact of sodium silicate stabilization on brightness

Development in thermomechanical pulp (TMP) bleaching.

The amount of caustic soda applied correlates directly with the effluent load

[35]. Taking the data from Fig. 6.2 and plotting the input of caustic soda against

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